Three-dimensional radiative transfer problems

MYSTIC - the Monte Carlo code for the physically
correct tracing of photons in cloudy atmospheres - is a powerful three-dimensional
radiative transfer solver which evolved from a very simple, one-layer,
one-dimensional, Monte Carlo code
[Mayer et al., 1998].
MYSTIC
handles broken clouds, inhomogeneous surface albedo, and topography.
The model is interfaced similar to DISORT
[Stamnes et al., 1988].
Currently, it is driven either by libRadtran
or TUV which provide a convenient way to set
up a one-dimensional atmosphere. Three-dimensional clouds, surface albedo, altitude,
and output resolution are specified in separate files in a format similar to
the SHDOM input.

Selected Applications

Radiation during a solar eclipse: A solar eclipse is not
only a spectacular phenomenon but also one of the most three-dimensional radiative transfer
problems one can imagine: Photons reaching the shadow under a total eclipse get there
exclusively by horizontal photon transport. Under cloudless conditions, a solar eclipse
is therefore an ideal experimental test for a three-dimensional radiative transfer code.
[Emde and Mayer, 2007] implemented
backward photon tracing and spherical geometry in the MYSTIC model to allow the first
exact simulation of radiation during a solar eclipse. The agreement with observations
is nearly perfect [Kazantzidis et al., 2007]

Remote sensing of inhomogeneous clouds: Clouds are inherently inhomogeneous
at all spatial scales. Cloud remote sensing relies on two basic assuptions: 1, that
clouds are homogeneous over the field-of-view of the satellite (the "plane-parallel assumption");
and 2, that horizontal photon transport can be neglected, or equivalently, that
individual satellite pixels can be treated independently of each other (the "independent pixel
approximation"). Both assumtions introduce bias and random noise into the retrieved cloud
properties. See a nice example of radiance reflected by an
inhomogeneous (cubic) clouds field, compared to the independent pixel assumption.
In a systematic study we quantified this uncertainty for different sensors
[Zinner et al., 2006],
[Zinner and Mayer, 2006].

Photon pathlength in clouds: This is a straightforward application
because the photon pathlength distribution is a natural by-product
of a Monte Carlo model. The average pathlength can also be calculated using any
radiative transfer model. Examples and comparisons between both approaches are shown
by [Mayer et al., 1998].
This work was the birth of MYSTIC!.

Effects of inhomogeneous surface albedo and topography: Many measurement sites
are located at places which are far from the ideal (flat and homogeneous) area which
is assumed by a one-dimensional model. A typical example is the site of
Tromsoe:
in winter, the land is covered with snow while the sea is typically snow free.
Due to the complicated coast line and the surrounding mountains, the interpretation
of the measurements at Tromsoe requires the application of a three-dimensional model
capable of handling inhomogeneous surface albedo and topography, see
[Kylling et al., 1999].
For some new applications have a look at the
simulations for Schneefernerhaus, the Metcrax experiment, or
for Weissfluhjoch next to Davos. These
calcultions are based on a high-resolution topography and illustrate the new
backward photon tracing capabilities of MYSTIC.

Radiative transfer in inhomogeneous clouds: There are basically two different approaches to
the investigation of inhomogeneous clouds. One is the study of realistic situations,
using as many measured properties of clouds as possible (high-resolution satellite
measurements, combined with sky photographs, etc.). A different approach are
statistical investigations, using well-established properties of clouds, like
their fractal dimension, size distribution, etc. An example is shown
here. More realistic examples are shown
on the MYSTIC page.